a) Field of the Invention
The invention is directed to a projection unit for a head-up display comprising an image generator, a mirror and a refractive lens which are arranged in a housing one behind the other in the light propagation direction, wherein the beam path is directed to a windshield. An arrangement of this kind is applied chiefly in automotive construction, but is not limited to this.
b) Description of the Related Art
EP 0 377 773 B1 shows a display device for a head-up display which contains an image generator, a mirror element and a transparent cover. The imaging characteristics are not mentioned. However, it can be gathered from the drawings that the mirror element is plane and the cover is plane-parallel and curved in a convex manner toward the windshield.
DE 695 06 174 T2 describes a head-up display which uses an image generator and an aspheric mirror. The imaging characteristics are not mentioned.
EP 0 643 315 B1 describes a head-up display comprising an image generator, a plane deflecting mirror, and a lens. The lens covers the dashboard scoop of a motor vehicle. There is also no mention here of the imaging characteristics.
Further, DE 69120575T2, EP 0486165A1 and EP 1291701A1 describe other arrangements for a head-up display which can be realized with two or more mirrors. It has been shown in practice that the use of a plurality of mirrors requires considerable installation space, so that the requirements can only be met with great difficulty particularly for applications in passenger vehicles.
The principal function of optical elements in a head-up display is to transform the size of the image generator into the size of the desired image by means of optical imaging and/or to fold the beam path.
The light coming from an image-generating element reaches the eyes of the driver by way of at least two mirrors and the reflection at the windshield of the automobile and generates an image at a distance of approximately 2 m to 3 m in front of the driver which is located slightly below the actual viewing direction adopted when driving. In this way, it is possible to view the environment and the reflected information at the same time in a relaxed manner. Imaging of this kind has the following essential quality features:                image field size        eye box size (by which is meant the region at eye level from which the driver can still see the image when moving his/her head)        absence of distortion        resolution        brightness        
Since the windshield (WSS) does not have a typically regular surface shape but is also used for imaging, it is necessary to include a correction of the imaging error of the WSS. When using a maximum of four mirrors, this can only be achieved in that some mirrors must be constructed as free-form mirrors. Therefore, the simplest systems of this type comprise at least two free-form mirrors. However, manufacture of free-form mirrors is very cost-intensive and, since reflections are involved, they are also particularly susceptible to tolerances relating to surface shape and accuracy of installation.